This invention relates to ink jet printing devices. More particularly, the present invention relates to a hybrid hot melt/thermal ink jet printhead having an array of coplanar nozzles in a nozzle face.
Thermal ink jet printing is a type of drop-on-demand ink jet system wherein an ink jet printhead expels ink droplets by the selective application of a current pulse to a thermal energy generator, usually a resistor, located in an ink channel a predetermined distance upstream from a channel nozzle or orifice. Usually, a plurality of capillary filled parallel ink channels are provided in the ink jet printing device. The channel ends opposite the respective nozzles are in communication with an ink reservoir to which an external ink supply is connected. The current pulses momentarily vaporize the ink and form bubbles on demand. Each temporary bubble expels an ink droplet and propels it towards a recording medium.
Such a printhead may be incorporated into either a carriage type printer or a pagewidth type printer. A carriage type printer generally has a relatively small printhead containing the ink channels and nozzles. The printhead is usually sealingly attached to a disposable ink supply cartridge in a combined printhead and cartridge assembly. This is reciprocated to print one swath of information at a time on a stationarily held recording medium, such as paper. After the swath of information is printed, the paper is stepped a distance equal to the height of the printed swath so that the next printed swath will be contiguous therewith. The procedure is repeated until the entire page is printed. In contrast, the pagewidth printer has a stationary printhead having a length equal to or greater than the width of the paper. The paper is continually moved past the printhead in a direction normal to the printhead at a constant speed during the printing process.
Such thermal ink jet printheads are known. One particularly advantageous thermal ink jet printhead is disclosed in U.S. Pat. No. 5,132,707 dated Jul. 21, 1992. That patent is incorporated herein by reference, in its entirety.
Thermal ink jet systems use inks prepared with water or another vaporizable solvent. Such inks require drying (i.e. vaporization of the solvent) after the ink has been applied to a substrate such as paper. In the absence of drying, the ink would smear on the paper and/or soak into the paper making the information represented by the ink difficult to comprehend.
Hot melt ink jets are similar to the thermal ink jets described above. However, a hot melt ink, contains no solvent. Thus, rather than being liquid at room temperature, a hot melt ink is typically a solid or semi-solid having a wax-like consistency. Such inks usually need to be heated to approximately 100.degree. C. before the ink melts and turns into a liquid. As with the thermal ink jet, a plurality of ink jet nozzles are provided in a printhead. A piezoelectric vibrating element is located in each ink channel upstream from a nozzle so that the piezoelectric oscillations propel ink through the nozzle. After the hot melt ink is applied to the substrate, the ink is resolidified by freezing on the substrate.
Each of these types of known ink jets, however, has its own advantages and disadvantages. One advantage of thermal ink jets is their compact design for the integrated electronics section of the printhead. Thermal ink jets are disadvantageous in that the thermal ink has a tendency to soak into a plain paper medium. This blurs the print or thins out the print locally thereby adversely affecting print quality. Problems have been encountered with thermal ink jets in attempting to rid the ink of moisture fast enough so that the ink does not soak into a plain paper medium. This is particularly true when printing with color. Therefore, usually when printing with thermal ink, one needed to use coated papers, which are more expensive than plain paper.
One advantage of a hot melt ink jet is its ability to print on plain paper since the hot melt ink quickly solidifies as it cools and, since it is waxy in nature, does not normally soak into a paper medium. However, hot melt ink jets are cumbersome in structure and in design. That is, the associated integrated electronics of a thermal ink jet head are considerably more compact than those of a hot melt ink jet head.
Accordingly, it has been considered desirable to develop a new and improved hybrid thermal/hot melt ink jet printhead which would combine the advantages of a thermal ink jet with the advantages of a hot melt ink jet and avoid the disadvantages of both in a novel structure that would overcome the foregoing difficulties and others while providing better and more advantageous overall results.